McMahon studied English and Drama at University of Bristol. She later qualified as a teacher of English and Drama and taught at a Hertfordshire comprehensive school. She taught writing skills with the Royal Literary Fund at the University of Hertfordshire and University of Warwick, and was made a Fellow at the University of Hertfordshire in 2001-3. She is currently an Advisory Fellow. In her report, What's Going on with Students' Writing?(2004) McMahon explored the state of writing in British universities.

McMahon also trained as a magistrate in Hertfordshire and has designed training courses for magistrates. Additionally, she has written and performed in local theatre, the Abbey Theatre in St Albans, her last role was in Wilde's The Importance of Being Earnest. She has also performed at the Edinburgh Festival and appeared on BBC Radio 4 in Woman's Hour. McMahon has also worked as a lyricist on various projects.

• A Way through the Woods Sinclair Stevenson 1989, Bantam Books 1992 ISBN 0553404318
• Footsteps Flamingo 1997, paperback 1998 ISBN 0006550371
• Confinement Flamingo 1998, paperback 1999 ISBN 0006550800
• After Mary Flamingo 2000, paperback 2001 ISBN 0006551556
• The Alchemist's Daughter Weidenfeld & Nicolson 2006 ISBN 0297850857
• The Rose of Sebastopol Weidenfeld & Nicolson 2007 ISBN 0753823748
• Footsteps (Re-released) Phoenix 2008 ISBN 0753825449
• The Crimson Rooms Weidenfeld & Nicolson 11 June 2009 ISBN 0297853384
• Confinement (Re-released) Phoenix 11 June 2009 ISBN 0753823748
• The Crimson Rooms (US release) Putnam Adult 1 February 2010 ISBN 0399156224

Future Releases

• The Crimson Rooms (US paperback release) Berkley Trade 4 January 2011 ISBN-10: 042523858